Inclement weather hurts Lake Tahoe's clarity level in 2017

The clarity of Lake Tahoe in 2017 is the lowest on record, according to date released by the Tahoe Environmental Research Center at the University of California, Davis.

The data suggests that historic drought follow by record-breaking precipitation and warm lake temperatures can be blamed for deteriorating clarity.

The average annual clarity level for 2017 was 59.7 feet which is a 9.5-foot decrease from the previous year. Mid-lake clarity levels can swing widely from season to season and year to year, and the five-year average lake clarity is approximately 70 feet.

In 2017, Lake Tahoe’s low clarity was primarily the result of two extreme climatic and hydrologic events — a perfect storm, so to speak,” said TERC Director Geoffrey Schladow, a professor of engineering at UC Davis. “The combination of arguably the most extreme drought period ending with the most extreme precipitation year produced the low clarity values seen. Measurements for 2018 have already shown a large improvement that are more in line with the long-term trend.

New pattern

Scientists say clarity is typically at its worst in the summer, and tends to improve during the fall and winter months.

Clarity values through mid-March were better than in many recent years. However, they failed to improve during fall and winter. Water Year 2017 (October 2016-September 2017) was California’s second wettest and Nevada’s seventh wettest in a 122-year record, with the Tahoe Basin seeing up to 300 percent of its normal precipitation. An additional early winter storm in November 2017 added an unusual amount of sediment that impacted fall clarity.

In 2017, heavy rain flushed sediment that had been accumulating in streambeds for five years during the drought into the lake. Algal concentrations have been shown to reduce clarity in past years, but those were not significantly higher in 2017 in comparison to other years.

Moving forward

Dozens of public and private partners at Lake Tahoe are working to reduce stormwater pollution from roads and urban areas to restore streams to prevent erosion that harms lake clarity. Partners are also working to restore natural wetlands and meadows that were displaced by past development and play an important role in filtering water before it enters the lake.

That work is keeping tens of thousands of pounds of fine sediment particle pollution out of Lake Tahoe each year, and the lake has been responding with an improved clarity trend over the past 20 years. The goal is to restore lake clarity back to its historic level of nearly 100 feet.